As I’m sure you know kittens normally open their eyes between 7 and 14 days. They don’t always open on the same day but normally will open within 48 hours of each other. On occasion kittens get an infection which reseals an open eye or stop ones from opening at all. If you carefully wipe around the eye and across the lid with a warm moist cloth or cotton ball, then if there is an infection then you should be able to see some puss. If this is the case then you need to take your kitten to the vet to ensure no long term eye damage is sustained. If the kittens eyes are not both open by 18 days I would advise a trip to the vet anyway as damage can be done to the long term vision of the cat if an eye does not receive any stimulus by 3-4 weeks.